1. Field of the Invention
The present invention relates to a nitride-based semiconductor device and a method of fabricating the same, and more particularly, it relates to a nitride-based semiconductor device having an electrode and a method of fabricating the same.
2. Description of the Background Art
A nitride-based semiconductor laser device has recently been expected as a light source for an advanced large capacity optical disk, and actively developed.
In general, an insulating sapphire substrate is employed for forming a nitride-based semiconductor laser device. When a nitride-based semiconductor layer is formed on the sapphire substrate, however, a large number of defects (dislocations) are disadvantageously formed in the nitride-based semiconductor layer due to large difference between the lattice constants of the sapphire substrate and the nitride-based semiconductor layer. Consequently, the characteristics of the nitride-based semiconductor laser device are disadvantageously reduced.
In this regard, a nitride-based semiconductor laser device employing a nitride-based semiconductor substrate such as a GaN substrate having small difference in lattice constant with respect to a nitride-based semiconductor layer is proposed in general.
FIG. 7 is a sectional view showing a conventional nitride-based semiconductor laser device employing an n-type GaN substrate 101. Referring to FIG. 7, nitride-based semiconductor layers (102 to 110) are grown on a Ga face ((HKLM) plane: M denotes a positive integer) to be improved in crystallinity in a process of fabricating the conventional nitride-based semiconductor laser device. A nitrogen face ((HKL-M) plane: M denotes a positive integer) of the n-type GaN substrate 101 having a wurtzite structure is employed as the back surface, so that an n-side electrode 112 is formed on this back surface of the n-type GaN substrate 101. The fabrication process for the conventional nitride-based semiconductor laser device is now described in detail.
As shown in FIG. 7, an n-type layer 102 consisting of n-type GaN having a thickness of about 3 xcexcm, an n-type buffer layer 103 consisting of n-type In0.05Ga0.95N having a thickness of about 100 nm, an n-type cladding layer 104 consisting of n-type Al0.05Ga0.95N having a thickness of about 400 nm, an n-type light guide layer 105 consisting of n-type GaN having a thickness of about 70 nm, an MQW (multiple quantum well) active layer 106 having an MQW structure, a p-type layer 107 consisting of p-type Al0.2Ga0.8N having a thickness of about 200 nm, a p-type light guide layer 108 consisting of p-type GaN having a thickness of about 70 nm, a p-type cladding layer 109 consisting of p-type Al0.05Ga0.95N having a thickness of about 400 nm and a p-type contact layer 110 consisting of p-type GaN having a thickness of about 100 nm are successively formed on the upper surface (Ga face) of the n-type GaN substrate 101 having a thickness of about 300 xcexcm to about 500 xcexcm.
Then, a p-side electrode 111 is formed on a prescribed region of the upper surface of the p-type contact layer 110. The back surface of the n-type GaN substrate 101 is polished until the thickness of the n-type GaN substrate 101 reaches a prescribed level of about 100 xcexcm, and an n-side electrode 112 is thereafter formed on the back surface (nitrogen face) of the n-type GaN substrate 101. Finally, the n-type GaN substrate 101 and the layers 102 to 110 are cleft thereby performing element isolation and forming a cavity facet. Thus, the conventional nitride-based semiconductor laser device shown in FIG. 7 is completed.
In the conventional nitride-based semiconductor laser device shown in FIG. 7, however, the n-type GaN substrate 101 is so hard that it is difficult to excellently perform device isolation and form the cavity facet by cleavage. In order to cope with such inconvenience, a method of mechanically polishing the back surface of the n-type GaN substrate 101 before the cleavage step for reducing irregularity on the back surface thereby excellently performing element isolation and forming the cavity facet is proposed. This method is disclosed in Japanese Patent Laying-Open No. 2002-26438, for example.
In the aforementioned conventional method disclosed in Japanese Patent Laying-Open No. 2002-26438, however, stress is applied in the vicinity of the back surface of the n-type GaN substrate 101 when the back surface of the n-type GaN substrate 101 is mechanically polished. Therefore, microscopic defects such as cracks are disadvantageously formed in the vicinity of the back surface of the n-type GaN substrate 101. Consequently, contact resistance between the n-type GaN substrate 101 and the n-side electrode 112 formed on the back surface (nitrogen face) thereof is disadvantageously increased.
Further, the nitrogen face of the n-type GaN substrate 101 is so easily oxidized that the contact resistance between the n-type GaN substrate 101 and the n-side electrode 112 formed on the back surface (nitrogen face) thereof is disadvantageously increased also by this.
An object of the present invention is to provide a method of fabricating a nitride-based semiconductor device capable of reducing contact resistance between the back surface of a nitride-based semiconductor substrate or the like and an electrode.
Another object of the present invention is to reduce the number of defects in the vicinity of the back surface of the nitride-based semiconductor substrate or the like in the aforementioned method of fabricating a nitride-based semiconductor device.
Still another object of the present invention is to provide a nitride-based semiconductor device capable of reducing contact resistance between the back surface of a nitride-based semiconductor substrate or the like and an electrode.
In order to attain the aforementioned objects, a method of fabricating a nitride-based semiconductor device according to a first aspect of the present invention comprises steps of etching the back surface of a first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure and thereafter forming an n-side electrode on the etched back surface of the first semiconductor layer.
In the method of fabricating a nitride-based semiconductor device according to the first aspect, the back surface of the first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure is etched as hereinabove described, whereby a region including defects in the vicinity of the back surface of the first semiconductor layer resulting from a polishing step or the like can be removed for reducing the number of defects in the vicinity of the back surface of the first semiconductor layer. Thus, an electron carrier concentration can be inhibited from reduction resulting from trap of electron carriers by defects, so that the electron carrier concentration can be increased on the back surface of the first semiconductor layer. Consequently, contact resistance between the first semiconductor layer and the n-side electrode can be reduced. Further, the back surface of the first semiconductor layer is so etched that flatness thereof can be improved as compared with that of a mechanically polished back surface. Thus, the n-side electrode formed on the back surface of the first semiconductor layer can also be improved in flatness, whereby adhesion between the n-side electrode and a radiator base can be improved when the former is mounted on the latter. Consequently, excellent radiability can be attained. Further, the n-side electrode formed on the back surface of the first semiconductor layer can be so improved in flatness that wire bondability with respect to the n-side electrode can be improved when the n-side electrode is wire-bonded.
In the aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect, the back surface of the first semiconductor layer preferably includes a nitrogen face of the first semiconductor layer. The term xe2x80x9cnitrogen facexe2x80x9d denotes a wide concept indicating not only a complete nitrogen face but also a surface mainly formed by a nitrogen face. More specifically, the term xe2x80x9cnitrogen facexe2x80x9d includes a surface having a nitrogen face of at least 50% in the present invention. When formed by a nitrogen face, the back surface of the first semiconductor layer is so easily oxidized that the oxidized portion of the back surface can be removed by etching. Thus, contact resistance between the first semiconductor layer and the n-side electrode can be further reduced.
In the aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect, the etching step preferably includes a step of etching the back surface of the first semiconductor layer by dry etching. According to this structure, the back surface of the first semiconductor layer can be easily improved in flatness and the number of defects can be reduced in the vicinity of the back surface due to the dry etching.
In the aforementioned method of fabricating a nitride-based semiconductor device including the step of etching the back surface of the first semiconductor layer by dry etching, the step of etching the back surface of the first semiconductor layer by dry etching preferably includes a step of etching the back surface of the first semiconductor layer by reactive ion etching with Cl2 gas and BCl3 gas. According to this structure, the back surface of the first semiconductor layer can be easily improved in flatness and the number of defects can be easily reduced in the vicinity of the back surface. In this case, the ratio of the flow rate of BCl3 gas to the flow rate of Cl2 gas in the step of etching the back surface of the first semiconductor layer by the reactive ion etching is preferably at least 30% and not more than 70%. It has been experimentally confirmed that the back surface of the first semiconductor layer can be improved in flatness in this range of the ratio of the flow rate of BCl3 gas to that of Cl2 gas, and hence the back surface of the first semiconductor layer can be reliably improved in flatness by setting the ratio within this range.
In the aforementioned method of fabricating a nitride-based semiconductor device including the step of etching the back surface of the first semiconductor layer by dry etching, the etching depth and the etching time in the step of etching the back surface of the first semiconductor layer by dry etching are preferably proportional to each other. According to this structure, the etching depth can be accurately controlled by adjusting the etching time.
In the aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect, the etching step preferably includes a step of etching the back surface of the first semiconductor layer thereby converting the back surface of the first semiconductor layer to a mirror surface. According to this structure, the back surface of the first semiconductor layer can be further improved in flatness.
The aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect preferably further comprises a step of performing heat treatment after the step of forming the n-side electrode. According to this structure, contact resistance between the first semiconductor layer and the n-side electrode can be further reduced.
In the aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect, the etching step preferably includes a step of etching the back surface of the first semiconductor layer by a thickness of at least about 1 xcexcm. According to this structure, a region including defects in the vicinity of the back surface of the first semiconductor layer resulting from a polishing step or the like can be so sufficiently removed that the number of defects can be further reduced in the vicinity of the back surface of the first semiconductor layer.
In the aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect, the first semiconductor layer may include the n-type nitride-based semiconductor layer or the nitride-based semiconductor substrate consisting of at least one material selected from a group consisting of GaN, BN, AlN, InN and TlN. Further, the n-side electrode may include an Al film.
In the aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect, the nitride-based semiconductor device is preferably a nitride-based semiconductor light-emitting device. According to this structure, contact resistance between the first semiconductor layer and the n-side electrode can be reduced in the nitride-based semiconductor light-emitting device, whereby the nitride-based semiconductor light-emitting device can attain excellent emissivity.
The aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect preferably further comprises a step of dipping a nitrogen face of the etched first semiconductor layer in a solution containing at least one of chlorine, fluorine, bromine, iodine, sulfur and ammonium in advance of the step of forming the n-side electrode. According to this structure, residues resulting from etching can be easily removed from the back surface of the first semiconductor layer. Thus, contact resistance between the first semiconductor layer and the n-side electrode can be further reduced. In this case, the method of fabricating a nitride-based semiconductor device further comprises a step of performing hydrochloric acid treatment on the back surface of the first semiconductor layer with an HCl solution in advance of the step of forming the n-side electrode. According to this structure, chlorine-based residues adhering to the back surface of the first semiconductor layer due to the etching can be easily removed.
The aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect preferably further comprises a step of polishing the back surface of the first semiconductor layer in advance of the etching step. Also when polishing the back surface of the first semiconductor layer, the back surface of the first semiconductor layer can be improved in flatness and the number of defects resulting from polishing can be reduced in the vicinity of the back surface through the etching step following the polishing step.
In the aforementioned method of fabricating a nitride-based semiconductor device according to the first aspect, the etching step preferably includes a step of etching the back surface of the first semiconductor layer by wet etching. According to this structure, the back surface of the first semiconductor layer can be easily improved in flatness and the number of defects can be easily reduced in the vicinity of the back surface due to the wet etching. In this case, the step of etching the back surface of the first semiconductor layer by wet etching preferably includes a step of etching the back surface of the first semiconductor layer with at least one etchant selected from a group consisting of aqua regia, KOH and K2S2O8. Further, the step of etching the back surface of the first semiconductor layer by wet etching preferably includes a step of etching the back surface of the first semiconductor layer while increasing the temperature to about 120xc2x0 C. According to this structure, the etching rate can be increased to about 10 times that in wet etching carried out under the room temperature.
A method of fabricating a nitride-based semiconductor device according to a second aspect of the present invention comprises steps of etching a nitrogen face of a first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure by dry etching and thereafter forming an n-side electrode on the etched nitrogen face of the first semiconductor layer.
In the method of fabricating a nitride-based semiconductor device according to the second aspect, the nitrogen face of the first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure is etched by dry etching as hereinabove described, whereby a region including defects in the vicinity of the first semiconductor layer resulting from a polishing step or the like can be so reduced that the number of defects can be reduced in the vicinity of the nitrogen face of the first semiconductor layer. Thus, reduction of an electron carrier concentration resulting from trap of electron carriers by defects can be suppressed, whereby the electron carrier concentration can be increased in the nitrogen face of the first semiconductor layer. Consequently, contact resistance between the first semiconductor layer and the n-side electrode can be reduced. Further, the nitrogen face of the first semiconductor layer is so etched by dry etching that flatness thereof can be improved as compared with that of a mechanically polished nitrogen face. Thus, the n-side electrode formed on the nitrogen face of the first semiconductor layer can also be improved in flatness, whereby adhesion between the n-side electrode and a radiator base can be improved when the former is mounted on the latter. Consequently, high radiability can be attained. Further, the n-side electrode formed on the nitrogen face of the first semiconductor layer can be so improved in flatness that wire bondability with respect to the n-side electrode can be improved when the n-side electrode is wire-bonded.
A nitride-based semiconductor device according to a third aspect of the present invention is formed through steps of etching the back surface of a first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure and thereafter forming an n-side electrode on the etched back surface of the first semiconductor layer.
In the nitride-based semiconductor device according to the third aspect, a region including defects in the vicinity of the first semiconductor layer resulting from a polishing step or the like can be removed by etching the back surface of the first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure as hereinabove described, whereby the number of defects can be reduced in the vicinity of the back surface of the first semiconductor layer. Thus, an electron carrier concentration can be inhibited from reduction resulting from trap of electron carriers by defects, whereby the electron carrier concentration can be increased on the back surface of the first semiconductor layer. Consequently, contact resistance between the first semiconductor layer and the n-side electrode can be reduced. Further, the back surface of the first semiconductor layer is so etched that flatness thereof can be improved as compared with that of a mechanically polished back surface. Thus, the n-side electrode formed the back surface of the first semiconductor layer can also be improved flatness, whereby adhesion between the n-side electrode and a radiator base can be improved when the former is mounted on the latter. Further, the n-side electrode formed on the back surface of the first semiconductor layer can be so improved in flatness that wire bondability with respect to the n-side electrode can be improved when the n-side electrode is wire-bonded.
A nitride-based semiconductor device according to a fourth aspect of the present invention comprises a first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure and an n-side electrode formed on the back surface of the first semiconductor layer, while contact resistance between the n-side electrode and the first semiconductor layer is not more than 0.05 xcexa9cm2.
In the nitride-based semiconductor device according to the fourth aspect, the contact resistance between the n-side electrode and the first semiconductor layer is set to not more than 0.05 xcexa9cm2, so that the nitride-based semiconductor device can attain excellent device characteristics by reducing the contact resistance between the n-side electrode and the first semiconductor layer.
In the aforementioned nitride-based semiconductor device according to the fourth aspect, an electron carrier concentration is preferably at least 1xc3x971017 cmxe2x88x923 in the vicinity of the interface between the first semiconductor layer and the n-side electrode. According to this structure, the nitride-based semiconductor device can easily reduce the contact resistance between the n-side electrode and the first semiconductor layer.
In the aforementioned nitride-based semiconductor device according to the fourth aspect, a dislocation density is preferably not more than 1xc3x97109 cmxe2x88x922 in the vicinity of the interface between the first semiconductor layer and the n-side electrode. According to this structure, the number of defects (dislocations) can be reduced in the vicinity of the interface between the first semiconductor layer and the n-side electrode, whereby the contact resistance can be reduced in the interface between the first semiconductor layer and the n-side electrode.
In the aforementioned nitride-based semiconductor device according to the fourth aspect, the back surface of the first semiconductor layer preferably includes a nitrogen face of the first semiconductor layer.
In the aforementioned nitride-based semiconductor device according to the fourth aspect, the first semiconductor layer may include the n-type nitride-based semiconductor layer or the nitride-based semiconductor substrate consisting of at least one material selected from a group consisting of GaN, BN, AlN, InN and TlN. Further, the n-side electrode may include an Al film.
In the aforementioned nitride-based semiconductor device according to the fourth aspect, the nitride-based semiconductor device is preferably a nitride-based semiconductor light-emitting device. According to this structure, contact resistance between the first semiconductor layer and the n-side electrode can be reduced in the nitride-based semiconductor light-emitting device, whereby the nitride-based semiconductor light-emitting device can attain excellent emissivity.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.